The field to which this invention is applicable is that of electrical discharge machining, sometimes hereinafter referred to as "EDM" in which process material is removed from an electrically conductive workpiece by erosive electrical discharges passed across a dielectric filled gap from a tool electrode. The process is precisely controlled, usually by the use of an independent pulse generator and one or more electronic output switches in an output module. The output module switches are periodically turned on and off at a preset frequency to connect and disconnect a DC power supply from the gap thus to provide machining power pulses. In some types of machining operations, it is desirable to use a higher cutting voltage than that of a standard power module. In standard EDM operations, the power supply voltage may be of the level of 70-80 volts. When high voltage or hi pol machining is used, the voltage level is raised to as high as 180 or 200 volts particularly for finishing operations. It is important that the power supply have the capibility of being operated in either mode for both roughing and finishing operations.